Greenhouse gas emissions, and particularly carbon dioxide emissions, are an increasing concern in the context of environmental health and as a contributor to global climate change. As such, much effort is currently spent developing ways to reduce or eliminate carbon dioxide emissions from important modern processes, particularly industrial processes.
A well-known and predominant source of carbon dioxide emissions in the modern industrial economy arises from power production. Accordingly, this sector has been the focus of efforts to reduce or eliminate carbon dioxide emissions. One typical approach to accomplish this objective is the use of a separation membrane or amine-based separation solution to capture carbon dioxide after combustion but prior to releasing exhaust into the atmosphere. Some solutions instead attempt to capture carbon dioxide pre-combustion, but have met with limited success, and are associated with prohibitively high costs due to the need to integrate legacy systems with complex fuel-conversion processes.
Focusing now on post-combustion capture techniques, many conventional separation systems employ a solid phase membrane, such as a polymer or ceramic; a solution of organic amines (e.g. monoethanolarnine, or MEA) solvated in water; or a dual phase molten carbonate/ceramic membrane to accomplish separation of carbon dioxide.
Typical polymer-based membrane separation systems generally separate gases through size effects or chemical effects. Size exclusion membranes effectively separate carbon dioxide from hydrocarbons but have prohibitively low selectivity for separating carbon dioxide from other gases, such as molecular nitrogen, as needed for the remediation of flue gases common in the targeted application to industrial sources of greenhouse gas emissions. Chemical exclusion membranes separate carbon dioxide from molecular nitrogen with high selectivity, but at prohibitively slow rates. Polymer membranes are also troubled by limited temperature ranges <250 C) and are prone to fouling by gases (e.g. sulfur and nitrogen oxides) and/or particulate matter in the flue gas.
Conventional oxygen separation membranes employ solid-phase ceramic separation systems which separate molecular oxygen from molecular nitrogen before combustion, instead of separating carbon dioxide from molecular nitrogen after combustion. These membranes require operation at temperatures of 800-1000 C, which is much higher than the 300-700 C temperature range at which targeted sources of greenhouse gas emissions operate, the temperature at which flue gas exits the combustion chamber (prior to the heat exchanger). Preheating air to this high temperature range for removing the 20% fraction of molecular oxygen is energetically expensive, and reduces the efficiency of the power production capability of the emission source. This system is also reactively slow (1×10−11 to 1×10−8 mol s−1 cm−2 in the 800-1000 C range) because the oxygen must transition through the solid phase as oxide (O2−) ions.
Traditional liquid phase treatment, such as amine gas treatment involves using an organic amine in a water solvent to capture cold (<80 C) carbon dioxide in an absorber from the flue gas of the emission source, e.g. a fossil fuel power plant. The carbon dioxide is later released in a regenerator with the application of heat energy. This system consumes 25-40% of the power plant's energy through exchanged and consumed heat energy. The infrastructure costs for an absorber, regenerator, etc., are also substantial. Together, the energy and infrastructure costs for this system are expected to double the cost of electricity produced by emission source. Unfortunately, the use of other materials for carbon dioxide adsorption does not significantly change the energy or infrastructure costs.
Existing dual phase separation membranes, such as carbonate-ceramic membranes, separate carbon dioxide from molecular nitrogen using both a molten (liquid) carbonate phase and a solid ceramic phase. The solid ceramic phase serves both as a porous, solid structural support for the molten carbonate phase as well as an oxide ion conductor via the same conduction process involved with oxygen separation membranes. The molten carbonate phase transports carbon dioxide across the membrane as carbonate ions (CO32−), while oxide conduction in the opposite direction to carbonate conduction is required to maintain oxygen and charge balance. The conduction of O2− through the solid phase is much slower than the relatively fast conduction of carbonate through the liquid phase and so the whole system is rate-limited by the slow rate of oxide conduction.
While mixtures or eutectics of lithium, sodium, and potassium carbonate can reach melting temperatures as low as 400-500 C and can conduct carbonate ions in this temperature range, this system nonetheless requires much higher operational temperatures because of the additional requirement for oxide conduction. Similar to oxygen separation membranes, the application of the carbonate-ceramic dual phase separation membrane is limited to very high temperatures (>700 C) due to the substantial thermal energy required to move oxide ions through the solid phase. The concurrent problems with a dependence upon oxide conduction through a solid phase render the separation process prohibitively slow and requires operation temperatures which are too high for effective separation of carbon dioxide from hot flue gas.
Accordingly, it would be beneficial to provide systems and techniques for separating carbon dioxide gas from emissions sources such as flue pas of fossil fuel power plants that are capable of operating at the ambient temperature of the flue gas as emitted from the source with a sufficient reaction rate to effectively separate the carbon dioxide without consuming substantial power from the source and without associated infrastructure costs incurred by the existing techniques and systems described above.